The present invention relates generally to devices capable of determining the angular position of a moving element and more particularly to position detectors for accurate detection, measurement and control of the angular position of a galvanometer.
Precision measurements and control of the angular position of a rotating element, such as a shaft in a limited-rotation motor, are often required. For example, galvanometers used for electronic manufacturing and repair operations, in which a laser beam is directed to perform tasks such as the profiling, marking cutting, drilling, and trimming of silicon, require high speed position sensors, which must detect the angular position of the rotor with great accuracy and repeatability, high temperature stability, and high signal to noise ratio. To enable high speeds of operation, these sensors must be small in size and have low inertia.
In the field of galvanometers, the precision requirements for angular position detection may be one part in one thousand or even one part in one million, depending upon the application. Presently, there are a number of different approaches used for sensing and detecting rotary motion and position of galvanometers including variable differential transformers, variable potentiometers, light detection systems, other optical devices, and capacitance sensing systems. Many of these detectors are acceptable for some types of uses but have drawbacks associated therewith or are unsatisfactory for other types of uses. The present invention relates to capacitance sensing devices.
The prior art primarily uses two types of capacitive position detectors. The first design includes a rotor in the form of a metallic cylindrical extension of the galvanometer shaft having circumferentially spaced xe2x80x9cfingers.xe2x80x9d The rotor is disposed within a concentric tubular plate, which is divided into a multiplicity of electrically separate stator elements symmetrically disposed about its circumference. Rotation of the shaft changes the relative capacitance between the rotor fingers and the elements. This design has substantial sensitivity to rotational motion, and thus has a high signal output per unit of rotation, but is defective in that it is also as sensitive to unintended radial motion.
Another design is a planar design, with a lobed dielectric rotor or xe2x80x9cbutterflyxe2x80x9d extending in a plane perpendicular to the axis of rotation, and interposed between two parallel metal plates, one an excitation electrode; the other, which is divided into a multiplicity of sensing sectors, serves as a signal plate. Rotation of the rotor changes the relative capacitance between the excitation electrode and individual sensing sections. This design avoids the sensitivity to radial motion from which the first design suffers, but substitutes sensitivity to axial motion.
Furthermore, for equivalent sensitivity, the planar design has a substantially greater moment of inertia, and so has a poor figure of merit. Both of these prior designs suffer from undesirable parasitic current paths to ground, which both waste excitation power and produce unnecessary noise in the system. They also require that the rotor be grounded to the stator and that the stator be isolated from earth ground by the galvanometer user.
What is needed is a position detector that limits excitation loss and also has a relatively low moment of inertia.
The novel design of the present invention combines the moving-dielectric of the planar design with the concentric geometry of the earlier moving-metal design. A central cylindrical pin, fixed to the stationary part of the galvanometer, acts as an excitation xe2x80x9cplate.xe2x80x9d A concentric segmented tubular signal plate, which is fixed to the stationary part of the scanner, acts as a sensing xe2x80x9cplate.xe2x80x9d Mounted to the rotor, and extending into the annular space between the excitation pin and the segmented tubular signal plate is a lobed cylindrical dielectric. This geometry has an area and spacing, and thus a sensitivity, comparable with the planar geometry, but because the dielectric is arranged in a tubular shape rather than a planar shape, its radius is much smaller than the average radius of an equivalent butterfly shaped dielectric, and so the corresponding moment of inertia is less. This results in a substantial improvement in the figure of merit.
This sensor geometry also provides considerable advantage with respect to the excitation efficiency. By virtue of the cylindrical geometry, the oscillator excites a volume almost fully enclosed by the sensor, so that little of the excitation energy is lost; the radiated RF energy is greatly reduced as well.